1. Field of the Invention
The present invention relates to a switching power supply, and particularly to a control circuit for a self-exciting DC/DC converter.
2. Description of the Related Art
For the purpose of generating higher voltage than the input voltage, step-up switching power supplies are widely used in various electronic devices. Such a step-up switching power supply includes a switching element and an inductor or a transformer. With such an arrangement, the switching element is alternately turned on and off in a time division manner so as to generate back electromotive force in the inductor or the transformer, thereby boosting the input voltage, i.e., thereby outputting voltage that has been stepped up.
With an insulated DC/DC converter, when a switching transistor is turned on, an electrical current flows through the primary winding of the transformer, thereby storing energy in the transformer. Then, when the switching transistor is turned off, the energy thus stored in the transformer is transferred as a charging current from the secondary winding of the transformer to an output capacitor via a rectifier diode, thereby generating output voltage that has been stepped up. After the energy stored in the transformer has been transferred to the output capacitor, the current that flows through the rectifier diode becomes zero.
A self-exciting DC/DC converter, which is a kind of insulated DC/DC converter, is known, which has a configuration that does not involve an oscillator, and which has a function in which the primary winding or the secondary winding of a transformer is monitored, and on/off control is performed for the switching transistor according to the state of the primary winding or the secondary winding of the transformer thus monitored (see Patent documents 1 and 2).    [Patent Document 1]
Japanese Patent Application Laid-open No. 2004-201474    [Patent Document 2]
Japanese Patent Application Laid-open No. 2005-73483
Here, let us consider a control method for a self-exciting DC/DC converter, in which the currents that flow through the primary winding and the secondary winding of the transformer are monitored. With such an arrangement, the switching transistor is turned on at the timing when the current Ic2 that flows through the secondary winding becomes zero. On the other hand, the switching transistor is turned off at the timing when the current Ic1 that flows through the primary winding reaches a predetermined level Ith1.
With such a control method, a first resistor is connected to the primary winding of the transformer with the electric potential of one end of the first resistor having been set to a fixed value. Also, a second resistor is connected to the secondary winding of the transformer, with the electric potential of one end of the second resistor having been set to a fixed value. With such an arrangement, the currents that flow through the primary winding and the secondary winding are converted into respective voltages. Furthermore, first and second voltage comparators are provided to comparing the voltages thus converted with a first threshold voltage Vth1 and a second threshold voltage Vth2, respectively. With such an arrangement, the ON/OFF state of the switching transistor is controlled according to the outputs from the two voltage comparators.
Here, the upper limit value Ith1 of the current that flows through the primary winding (which will be referred to as the “first primary current Ic1” hereafter) is determined based upon the resistance value of the first resistor R1 and the first threshold voltage Vth, i.e., it can be obtained using the expression Ith1=R1×Vth1. Accordingly, irregularities in the resistance value of the first resistor R1 lead to irregularities in the peak value of the first current Ic1. This leads to irregularities in the switching transistor ON-time, which affects the step-up voltage operation. Let us consider an arrangement in which the first resistor R1 is integrally formed on a semiconductor substrate. In particular, such an arrangement exacerbates the aforementioned problem.